在分析源码之前，我们先分析，哪些地方用到了EventLoop？
NioServerSocketChannel的连接监听注册
NioSocketChannel的IO事件注册
NioServerSocketChannel连接监听#
在AbstractBootstrap类的initAndRegister()方法中，当NioServerSocketChannel初始化完成后，会调用case标记位置的代码进行注册。
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
channel = channelFactory.newChannel();
init(channel);
} catch (Throwable t) {
}
//注册到boss线程的selector上。
ChannelFuture regFuture = config().group().register(channel);
if (regFuture.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
}
return regFuture;
}AbstractNioChannel.doRegister#
按照代码的执行逻辑，最终会执行到AbstractNioChannel的doRegister()方法中。
@Override
protected void doRegister() throws Exception {
boolean selected = false;
for (;;) {
try {
//调用ServerSocketChannel的register方法，把当前服务端对象注册到boss线程的selector上
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
if (!selected) {
// Force the Selector to select now as the "canceled" SelectionKey may still be
// cached and not removed because no Select.select(..) operation was called yet.
eventLoop().selectNow();
selected = true;
} else {
// We forced a select operation on the selector before but the SelectionKey is still cached
// for whatever reason. JDK bug ?
throw e;
}
}
}
}NioEventLoop的启动过程#
NioEventLoop是一个线程，它的启动过程如下。
在AbstractBootstrap的doBind0方法中，获取了NioServerSocketChannel中的NioEventLoop，然后使用它来执行绑定端口的任务。
private static void doBind0(
final ChannelFuture regFuture, final Channel channel,
final SocketAddress localAddress, final ChannelPromise promise) {
//启动
channel.eventLoop().execute(new Runnable() {
@Override
public void run() {
if (regFuture.isSuccess()) {
channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
} else {
promise.setFailure(regFuture.cause());
}
}
});
}SingleThreadEventExecutor.execute#
然后一路执行到SingleThreadEventExecutor.execute方法中，调用startThread()方法启动线程。
private void execute(Runnable task, boolean immediate) {
boolean inEventLoop = inEventLoop();
addTask(task);
if (!inEventLoop) {
startThread(); //启动线程
if (isShutdown()) {
boolean reject = false;
try {
if (removeTask(task)) {
reject = true;
}
} catch (UnsupportedOperationException e) {
// The task queue does not support removal so the best thing we can do is to just move on and
// hope we will be able to pick-up the task before its completely terminated.
// In worst case we will log on termination.
}
if (reject) {
reject();
}
}
}
if (!addTaskWakesUp && immediate) {
wakeup(inEventLoop);
}
}startThread#
private void startThread() {
if (state == ST_NOT_STARTED) {
if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
boolean success = false;
try {
doStartThread(); //执行启动过程
success = true;
} finally {
if (!success) {
STATE_UPDATER.compareAndSet(this, ST_STARTED, ST_NOT_STARTED);
}
}
}
}
}接着调用doStartThread()方法，通过executor.execute执行一个任务，在该任务中启动了NioEventLoop线程
private void doStartThread() {
assert thread == null;
executor.execute(new Runnable() { //通过线程池执行一个任务
@Override
public void run() {
thread = Thread.currentThread();
if (interrupted) {
thread.interrupt();
}
boolean success = false;
updateLastExecutionTime();
try {
SingleThreadEventExecutor.this.run(); //调用boss的NioEventLoop的run方法，开启轮询
}
//省略....
}
});
}NioEventLoop的轮询过程#
当NioEventLoop线程被启动后，就直接进入到NioEventLoop的run方法中。
protected void run() {
int selectCnt = 0;
for (;;) {
try {
int strategy;
try {
strategy = selectStrategy.calculateStrategy(selectNowSupplier, hasTasks());
switch (strategy) {
case SelectStrategy.CONTINUE:
continue;
case SelectStrategy.BUSY_WAIT:
case SelectStrategy.SELECT:
long curDeadlineNanos = nextScheduledTaskDeadlineNanos();
if (curDeadlineNanos == -1L) {
curDeadlineNanos = NONE; // nothing on the calendar
}
nextWakeupNanos.set(curDeadlineNanos);
try {
if (!hasTasks()) {
strategy = select(curDeadlineNanos);
}
} finally {
// This update is just to help block unnecessary selector wakeups
// so use of lazySet is ok (no race condition)
nextWakeupNanos.lazySet(AWAKE);
}
// fall through
default:
}
} catch (IOException e) {
// If we receive an IOException here its because the Selector is messed up. Let's rebuild
// the selector and retry. https://github.com/netty/netty/issues/8566
rebuildSelector0();
selectCnt = 0;
handleLoopException(e);
continue;
}
selectCnt++;
cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
boolean ranTasks;
if (ioRatio == 100) {
try {
if (strategy > 0) {
processSelectedKeys();
}
} finally {
// Ensure we always run tasks.
ranTasks = runAllTasks();
}
} else if (strategy > 0) {
final long ioStartTime = System.nanoTime();
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
final long ioTime = System.nanoTime() - ioStartTime;
ranTasks = runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
} else {
ranTasks = runAllTasks(0); // This will run the minimum number of tasks
}
if (ranTasks || strategy > 0) {
if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS && logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
selectCnt - 1, selector);
}
selectCnt = 0;
} else if (unexpectedSelectorWakeup(selectCnt)) { // Unexpected wakeup (unusual case)
selectCnt = 0;
}
} catch (CancelledKeyException e) {
// Harmless exception - log anyway
if (logger.isDebugEnabled()) {
logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",
selector, e);
}
} catch (Error e) {
throw (Error) e;
} catch (Throwable t) {
handleLoopException(t);
} finally {
// Always handle shutdown even if the loop processing threw an exception.
try {
if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
return;
}
}
} catch (Error e) {
throw (Error) e;
} catch (Throwable t) {
handleLoopException(t);
}
}
}
}NioEventLoop的执行流程#
NioEventLoop中的run方法是一个无限循环的线程，在该循环中主要做三件事情，如图9-1所示。
图9-1
轮询处理I/O事件（select），轮询Selector选择器中已经注册的所有Channel的I/O就绪事件
处理I/O事件，如果存在已经就绪的Channel的I/O事件，则调用processSelectedKeys进行处理
处理异步任务（runAllTasks），Reactor线程有一个非常重要的职责，就是处理任务队列中的非I/O任务，Netty提供了ioRadio参数用来调整I/O时间和任务处理的时间比例。
轮询I/O就绪事件#
我们先来看I/O时间相关的代码片段:
通过selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())获取当前的执行策略
根据不同的策略，用来控制每次轮询时的执行策略。
protected void run() {
int selectCnt = 0;
for (;;) {
try {
int strategy;
try {
strategy = selectStrategy.calculateStrategy(selectNowSupplier, hasTasks());
switch (strategy) {
case SelectStrategy.CONTINUE:
continue;
case SelectStrategy.BUSY_WAIT:
// fall-through to SELECT since the busy-wait is not supported with NIO
case SelectStrategy.SELECT:
long curDeadlineNanos = nextScheduledTaskDeadlineNanos();
if (curDeadlineNanos == -1L) {
curDeadlineNanos = NONE; // nothing on the calendar
}
nextWakeupNanos.set(curDeadlineNanos);
try {
if (!hasTasks()) {
strategy = select(curDeadlineNanos);
}
} finally {
// This update is just to help block unnecessary selector wakeups
// so use of lazySet is ok (no race condition)
nextWakeupNanos.lazySet(AWAKE);
}
// fall through
default:
}
}
//省略....
}
}
}selectStrategy处理逻辑#
@Override
public int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception {
return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT;
}如果hasTasks为true，表示当前NioEventLoop线程存在异步任务的情况下，则调用selectSupplier.get()，否则直接返回SELECT。
其中selectSupplier.get()的定义如下：
private final IntSupplier selectNowSupplier = new IntSupplier() {
@Override
public int get() throws Exception {
return selectNow();
}
};该方法中调用的是selectNow()方法，这个方法是Selector选择器中的提供的非阻塞方法，执行后会立刻返回。
如果当前已经有就绪的Channel，则会返回对应就绪Channel的数量
否则，返回0.
分支处理#
在上面一个步骤中获得了strategy之后，会根据不同的结果进行分支处理。
CONTINUE，表示需要重试。
BUSY_WAIT，由于在NIO中并不支持BUSY_WAIT，所以BUSY_WAIT和SELECT的执行逻辑是一样的
SELECT，表示需要通过select方法获取就绪的Channel列表，当NioEventLoop中不存在异步任务时，也就是任务队列为空，则返回该策略。
switch (strategy) {
case SelectStrategy.CONTINUE:
continue;
case SelectStrategy.BUSY_WAIT:
// fall-through to SELECT since the busy-wait is not supported with NIO
case SelectStrategy.SELECT:
long curDeadlineNanos = nextScheduledTaskDeadlineNanos();
if (curDeadlineNanos == -1L) {
curDeadlineNanos = NONE; // nothing on the calendar
}
nextWakeupNanos.set(curDeadlineNanos);
try {
if (!hasTasks()) {
strategy = select(curDeadlineNanos);
}
} finally {
// This update is just to help block unnecessary selector wakeups
// so use of lazySet is ok (no race condition)
nextWakeupNanos.lazySet(AWAKE);
}
// fall through
default:
}SelectStrategy.SELECT#
当NioEventLoop线程中不存在异步任务时，则开始执行SELECT策略
//下一次定时任务触发截至时间，默认不是定时任务，返回 -1L
long curDeadlineNanos = nextScheduledTaskDeadlineNanos();
if (curDeadlineNanos == -1L) {
curDeadlineNanos = NONE; // nothing on the calendar
}
nextWakeupNanos.set(curDeadlineNanos);
try {
if (!hasTasks()) {
//2. taskQueue中任务执行完，开始执行select进行阻塞
strategy = select(curDeadlineNanos);
}
} finally {
// This update is just to help block unnecessary selector wakeups
// so use of lazySet is ok (no race condition)
nextWakeupNanos.lazySet(AWAKE);
}select方法定义如下，默认情况下deadlineNanos=NONE，所以会调用select()方法阻塞。
private int select(long deadlineNanos) throws IOException {
if (deadlineNanos == NONE) {
return selector.select();
}
//计算select()方法的阻塞超时时间
long timeoutMillis = deadlineToDelayNanos(deadlineNanos + 995000L) / 1000000L;
return timeoutMillis <= 0 ? selector.selectNow() : selector.select(timeoutMillis);
}最终返回就绪的channel个数，后续的逻辑中会根据返回的就绪channel个数来决定执行逻辑。
NioEventLoop.run中的业务处理#
业务处理的逻辑相对来说比较容易理解
如果有就绪的channel，则处理就绪channel的IO事件
处理完成后同步执行异步队列中的任务。
另外，这里为了解决Java NIO中的空转问题，通过selectCnt记录了空转次数，一次循环发生了空转（既没有IO需要处理、也没有执行任何任务），那么记录下来（selectCnt）; ，如果连续发生空转（selectCnt达到一定值），netty认为触发了NIO的BUG（unexpectedSelectorWakeup处理）；
Java Nio中有一个bug，Java nio在Linux系统下的epoll空轮询问题。也就是在select()方法中，及时就绪的channel为0，也会从本来应该阻塞的操作中被唤醒，从而导致CPU 使用率达到100%。
@Override
protected void run() {
int selectCnt = 0;
for (;;) {
//省略....
selectCnt++;//selectCnt记录的是无功而返的select次数，即eventLoop空转的次数，为解决NIO BUG
cancelledKeys = 0;
needsToSelectAgain = false;
final int ioRatio = this.ioRatio;
boolean ranTasks;
if (ioRatio == 100) { //ioRadio执行时间占比是100%，默认是50%
try {
if (strategy > 0) { //strategy>0表示存在就绪的SocketChannel
processSelectedKeys(); //执行就绪SocketChannel的任务
}
} finally {
//注意，将ioRatio设置为100，并不代表任务不执行，反而是每次将任务队列执行完
ranTasks = runAllTasks(); //确保总是执行队列中的任务
}
} else if (strategy > 0) { //strategy>0表示存在就绪的SocketChannel
final long ioStartTime = System.nanoTime(); //io时间处理开始时间
try {
processSelectedKeys(); //开始处理IO就绪事件
} finally {
// io事件执行结束时间
final long ioTime = System.nanoTime() - ioStartTime;
//基于本次循环处理IO的时间，ioRatio，计算出执行任务耗时的上限，也就是只允许处理多长时间异步任务
ranTasks = runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
} else {
//这个分支代表：strategy=0，ioRatio<100，此时任务限时=0，意为：尽量少地执行异步任务
//这个分支和strategy>0实际是一码事，代码简化了一下而已
ranTasks = runAllTasks(0); // This will run the minimum number of tasks
}
if (ranTasks || strategy > 0) { //ranTasks=true，或strategy>0，说明eventLoop干活了，没有空转，清空selectCnt
if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS && logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
selectCnt - 1, selector);
}
selectCnt = 0;
}
//unexpectedSelectorWakeup处理NIO BUG
else if (unexpectedSelectorWakeup(selectCnt)) { // Unexpected wakeup (unusual case)
selectCnt = 0;
}
}
}processSelectedKeys#
通过在select方法中，我们可以获得就绪的I/O事件数量，从而触发执行processSelectedKeys方法。
private void processSelectedKeys() {
if (selectedKeys != null) {
processSelectedKeysOptimized();
} else {
processSelectedKeysPlain(selector.selectedKeys());
}
}处理I/O事件时，有两个逻辑分支处理：
一种是处理Netty优化过的selectedKeys，
另一种是正常的处理逻辑
processSelectedKeys方法中根据是否设置了selectedKeys来判断使用哪种策略，默认使用的是Netty优化过的selectedKeys，它返回的对象是SelectedSelectionKeySet。
processSelectedKeysOptimized#
private void processSelectedKeysOptimized() {
for (int i = 0; i < selectedKeys.size; ++i) {
//1. 取出IO事件以及对应的channel
final SelectionKey k = selectedKeys.keys[i];
selectedKeys.keys[i] = null;//k的引用置null，便于gc回收，也表示该channel的事件处理完成避免重复处理
final Object a = k.attachment(); //获取保存在当前channel中的attachment，此时应该是NioServerSocketChannel
//处理当前的channel
if (a instanceof AbstractNioChannel) {
//对于boss NioEventLoop，轮询到的基本是连接事件，后续的事情就是通过他的pipeline将连接扔给一个worker NioEventLoop处理
//对于worker NioEventLoop来说，轮循道的基本商是IO读写事件，后续的事情就是通过他的pipeline将读取到的字节流传递给每个channelHandler来处理
processSelectedKey(k, (AbstractNioChannel) a);
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
processSelectedKey(k, task);
}
if (needsToSelectAgain) {
// null out entries in the array to allow to have it GC'ed once the Channel close
// See https://github.com/netty/netty/issues/2363
selectedKeys.reset(i + 1);
selectAgain();
i = -1;
}
}
}processSelectedKey#
private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
if (!k.isValid()) {
final EventLoop eventLoop;
try {
eventLoop = ch.eventLoop();
} catch (Throwable ignored) {
}
if (eventLoop == this) {
// close the channel if the key is not valid anymore
unsafe.close(unsafe.voidPromise());
}
return;
}
try {
int readyOps = k.readyOps(); //获取当前key所属的操作类型
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {//如果是连接类型
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
if ((readyOps & SelectionKey.OP_WRITE) != 0) { //如果是写类型
ch.unsafe().forceFlush();
}
//如果是读类型或者ACCEPT类型。则执行unsafe.read()方法，unsafe的实例对象为 NioMessageUnsafe
if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
unsafe.read();
}
} catch (CancelledKeyException ignored) {
unsafe.close(unsafe.voidPromise());
}
}NioMessageUnsafe.read()#
假设此时是一个读操作，或者是客户端建立连接，那么代码执行逻辑如下，
@Override
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline(); //如果是第一次建立连接，此时的pipeline是ServerBootstrapAcceptor
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
allocHandle.incMessagesRead(localRead);
} while (continueReading(allocHandle));
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size();
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i)); //调用pipeline中的channelRead方法
}
readBuf.clear();
allocHandle.readComplete();
pipeline.fireChannelReadComplete();
if (exception != null) {
closed = closeOnReadError(exception);
pipeline.fireExceptionCaught(exception); //调用pipeline中的ExceptionCaught方法
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}SelectedSelectionKeySet的优化#
Netty中自己封装实现了一个SelectedSelectionKeySet，用来优化原本SelectorKeys的结构，它是怎么进行优化的呢？先来看它的代码定义
final class SelectedSelectionKeySet extends AbstractSet<SelectionKey> {
SelectionKey[] keys;
int size;
SelectedSelectionKeySet() {
keys = new SelectionKey[1024];
}
@Override
public boolean add(SelectionKey o) {
if (o == null) {
return false;
}
keys[size++] = o;
if (size == keys.length) {
increaseCapacity();
}
return true;
}
}SelectedSelectionKeySet内部使用的是SelectionKey数组，所有在processSelectedKeysOptimized方法中可以直接通过遍历数组来取出就绪的I/O事件。
而原来的Set<SelectionKey>返回的是HashSet类型，两者相比，SelectionKey[]不需要考虑哈希冲突的问题，所以可以实现O(1)时间复杂度的add操作。
SelectedSelectionKeySet的初始化#
netty通过反射的方式，把Selector对象内部的selectedKeys和publicSelectedKeys替换为SelectedSelectionKeySet。
原本的selectedKeys和publicSelectedKeys这两个字段都是HashSet类型，替换之后变成了SelectedSelectionKeySet。当有就绪的key时，会直接填充到SelectedSelectionKeySet的数组中。后续只需要遍历即可。
private SelectorTuple openSelector() {
final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
//使用反射
Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
try {
//Selector内部的selectedKeys字段
Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
//Selector内部的publicSelectedKeys字段
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
if (PlatformDependent.javaVersion() >= 9 && PlatformDependent.hasUnsafe()) {
//获取selectedKeysField字段偏移量
long selectedKeysFieldOffset = PlatformDependent.objectFieldOffset(selectedKeysField);
//获取publicSelectedKeysField字段偏移量
long publicSelectedKeysFieldOffset =
PlatformDependent.objectFieldOffset(publicSelectedKeysField);
if (selectedKeysFieldOffset != -1 && publicSelectedKeysFieldOffset != -1) {
//替换为selectedKeySet
PlatformDependent.putObject(
unwrappedSelector, selectedKeysFieldOffset, selectedKeySet);
PlatformDependent.putObject(
unwrappedSelector, publicSelectedKeysFieldOffset, selectedKeySet);
return null;
}
// We could not retrieve the offset, lets try reflection as last-resort.
}
Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField, true);
if (cause != null) {
return cause;
}
cause = ReflectionUtil.trySetAccessible(publicSelectedKeysField, true);
if (cause != null) {
return cause;
}
selectedKeysField.set(unwrappedSelector, selectedKeySet);
publicSelectedKeysField.set(unwrappedSelector, selectedKeySet);
return null;
} catch (NoSuchFieldException e) {
return e;
} catch (IllegalAccessException e) {
return e;
}
}
});
if (maybeException instanceof Exception) {
selectedKeys = null;
Exception e = (Exception) maybeException;
logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, e);
return new SelectorTuple(unwrappedSelector);
}
selectedKeys = selectedKeySet;
}异步任务的执行流程#
分析完上面的流程后，我们继续来看NioEventLoop中的run方法中，针对异步任务的处理流程
@Override
protected void run() {
int selectCnt = 0;
for (;;) {
ranTasks = runAllTasks();
}
}runAllTask#
需要注意，NioEventLoop可以支持定时任务的执行，通过nioEventLoop.schedule()来完成。
protected boolean runAllTasks() {
assert inEventLoop();
boolean fetchedAll;
boolean ranAtLeastOne = false;
do {
fetchedAll = fetchFromScheduledTaskQueue(); //合并定时任务到普通任务队列
if (runAllTasksFrom(taskQueue)) { //循环执行taskQueue中的任务
ranAtLeastOne = true;
}
} while (!fetchedAll);
if (ranAtLeastOne) { //如果任务全部执行完成，记录执行完完成时间
lastExecutionTime = ScheduledFutureTask.nanoTime();
}
afterRunningAllTasks();//执行收尾任务
return ranAtLeastOne;
}fetchFromScheduledTaskQueue#
遍历scheduledTaskQueue中的任务，添加到taskQueue中。
private boolean fetchFromScheduledTaskQueue() {
if (scheduledTaskQueue == null || scheduledTaskQueue.isEmpty()) {
return true;
}
long nanoTime = AbstractScheduledEventExecutor.nanoTime();
for (;;) {
Runnable scheduledTask = pollScheduledTask(nanoTime);
if (scheduledTask == null) {
return true;
}
if (!taskQueue.offer(scheduledTask)) {
// No space left in the task queue add it back to the scheduledTaskQueue so we pick it up again.
scheduledTaskQueue.add((ScheduledFutureTask<?>) scheduledTask);
return false;
}
}
}任务添加方法execute#
NioEventLoop内部有两个非常重要的异步任务队列，分别是普通任务和定时任务队列，针对这两个队列提供了两个方法分别向两个队列中添加任务。
execute()
schedule()
其中,execute方法的定义如下。
private void execute(Runnable task, boolean immediate) {
boolean inEventLoop = inEventLoop();
addTask(task); //把当前任务添加到阻塞队列中
if (!inEventLoop) { //如果是非NioEventLoop
startThread(); //启动线程
if (isShutdown()) { //如果当前NioEventLoop已经是停止状态
boolean reject = false;
try {
if (removeTask(task)) {
reject = true;
}
} catch (UnsupportedOperationException e) {
// The task queue does not support removal so the best thing we can do is to just move on and
// hope we will be able to pick-up the task before its completely terminated.
// In worst case we will log on termination.
}
if (reject) {
reject();
}
}
}
if (!addTaskWakesUp && immediate) {
wakeup(inEventLoop);
}
}Nio的空轮转问题#
所谓的空轮训，是指我们在执行selector.select()方法时，如果没有就绪的SocketChannel时，当前线程会被阻塞 。 而空轮询是指当没有就绪SocketChannel时，会被触发唤醒。
而这个唤醒是没有任何读写请求的，从而导致线程在做无效的轮询，使得CPU占用率较高。
导致这个问题的根本原因是：
在部分Linux的2.6的kernel中，poll和epoll对于突然中断的连接socket会对返回的eventSet事件集合置为POLLHUP，也可能是POLLERR，eventSet事件集合发生了变化，这就可能导致Selector会被唤醒。这是与操作系统机制有关系的，JDK虽然仅仅是一个兼容各个操作系统平台的软件，但很遗憾在JDK5和JDK6最初的版本中（严格意义上来将，JDK部分版本都是），这个问题并没有解决，而将这个帽子抛给了操作系统方，这也就是这个bug最终一直到2013年才最终修复的原因，最终影响力太广。
Netty是如何解决这个问题的呢？我们回到NioEventLoop的run方法中
@Override
protected void run() {
int selectCnt = 0;
for (;;) {
//selectCnt记录的是无功而返的select次数，即eventLoop空转的次数，为解决NIO BUG
selectCnt++;
//ranTasks=true，或strategy>0，说明eventLoop干活了，没有空转，清空selectCnt
if (ranTasks || strategy > 0) {
//如果选择操作计数器的值，大于最小选择器重构阈值，则输出log
if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS && logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
selectCnt - 1, selector);
}
selectCnt = 0;
}
//unexpectedSelectorWakeup处理NIO BUG
else if (unexpectedSelectorWakeup(selectCnt)) { // Unexpected wakeup (unusual case)
selectCnt = 0;
}
}
}unexpectedSelectorWakeup#
private boolean unexpectedSelectorWakeup(int selectCnt) {
if (Thread.interrupted()) {
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely because " +
"Thread.currentThread().interrupt() was called. Use " +
"NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
}
return true;
}
//如果选择重构的阈值大于0， 默认值是512次、 并且当前触发的空轮询次数大于 512次。，则触发重构
if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
// The selector returned prematurely many times in a row.
// Rebuild the selector to work around the problem.
logger.warn("Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",
selectCnt, selector);
rebuildSelector();
return true;
}
return false;
}rebuildSelector()#
public void rebuildSelector() {
if (!inEventLoop()) { //如果不是在eventLoop中执行，则使用异步线程执行
execute(new Runnable() {
@Override
public void run() {
rebuildSelector0();
}
});
return;
}
rebuildSelector0();
}rebuildSelector0#
这个方法的主要作用： 重新创建一个选择器,替代当前事件循环中的选择器
private void rebuildSelector0() {
final Selector oldSelector = selector; //获取老的selector选择器
final SelectorTuple newSelectorTuple; //定义新的选择器
if (oldSelector == null) { //如果老的选择器为空，直接返回
return;
}
try {
newSelectorTuple = openSelector(); //创建一个新的选择器
} catch (Exception e) {
logger.warn("Failed to create a new Selector.", e);
return;
}
// Register all channels to the new Selector.
int nChannels = 0;
for (SelectionKey key: oldSelector.keys()) {//遍历注册到选择器的选择key集合
Object a = key.attachment();
try {
//如果选择key无效或选择关联的通道已经注册到新的选择器，则跳出当前循环
if (!key.isValid() || key.channel().keyFor(newSelectorTuple.unwrappedSelector) != null) {
continue;
}
//获取key的选择关注事件集
int interestOps = key.interestOps();
key.cancel();//取消选择key
//注册选择key到新的选择器
SelectionKey newKey = key.channel().register(newSelectorTuple.unwrappedSelector, interestOps, a);
if (a instanceof AbstractNioChannel) {//如果是nio通道，则更新通道的选择key
// Update SelectionKey
((AbstractNioChannel) a).selectionKey = newKey;
}
nChannels ++;
} catch (Exception e) {
logger.warn("Failed to re-register a Channel to the new Selector.", e);
if (a instanceof AbstractNioChannel) {
AbstractNioChannel ch = (AbstractNioChannel) a;
ch.unsafe().close(ch.unsafe().voidPromise());
} else {
@SuppressWarnings("unchecked")
NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
invokeChannelUnregistered(task, key, e);
}
}
}
//更新当前事件循环选择器
selector = newSelectorTuple.selector;
unwrappedSelector = newSelectorTuple.unwrappedSelector;
try {
// time to close the old selector as everything else is registered to the new one
oldSelector.close(); //关闭原始选择器
} catch (Throwable t) {
if (logger.isWarnEnabled()) {
logger.warn("Failed to close the old Selector.", t);
}
}
if (logger.isInfoEnabled()) {
logger.info("Migrated " + nChannels + " channel(s) to the new Selector.");
}
}从上述过程中我们发现，Netty解决NIO空轮转问题的方式，是通过重建Selector对象来完成的，在这个重建过程中，核心是把Selector中所有的SelectionKey重新注册到新的Selector上，从而巧妙的避免了JDK epoll空轮训问题。
连接的建立及处理过程#
在9.2.4.3节中，提到了当客户端有连接或者读事件发送到服务端时，会调用NioMessageUnsafe类的read()方法。
public void read() {
assert eventLoop().inEventLoop();
final ChannelConfig config = config();
final ChannelPipeline pipeline = pipeline();
final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
allocHandle.reset(config);
boolean closed = false;
Throwable exception = null;
try {
try {
do {
//如果有客户端连接进来，则localRead为1，否则返回0
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
allocHandle.incMessagesRead(localRead); //累计增加read消息数量
} while (continueReading(allocHandle));
} catch (Throwable t) {
exception = t;
}
int size = readBuf.size(); //遍历客户端连接列表
for (int i = 0; i < size; i ++) {
readPending = false;
pipeline.fireChannelRead(readBuf.get(i)); //调用pipeline中handler的channelRead方法。
}
readBuf.clear(); //清空集合
allocHandle.readComplete();
pipeline.fireChannelReadComplete(); //触发pipeline中handler的readComplete方法
if (exception != null) {
closed = closeOnReadError(exception);
pipeline.fireExceptionCaught(exception);
}
if (closed) {
inputShutdown = true;
if (isOpen()) {
close(voidPromise());
}
}
} finally {
if (!readPending && !config.isAutoRead()) {
removeReadOp();
}
}
}pipeline.fireChannelRead(readBuf.get(i))#
继续来看pipeline的触发方法，此时的pipeline组成，如果当前是连接事件，那么pipeline = ServerBootstrap$ServerBootstrapAcceptor。
static void invokeChannelRead(final AbstractChannelHandlerContext next, Object msg) {
final Object m = next.pipeline.touch(ObjectUtil.checkNotNull(msg, "msg"), next);
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeChannelRead(m); //获取pipeline中的下一个节点，调用该handler的channelRead方法
} else {
executor.execute(new Runnable() {
@Override
public void run() {
next.invokeChannelRead(m);
}
});
}
}ServerBootstrapAcceptor#
ServerBootstrapAcceptor是NioServerSocketChannel中一个特殊的Handler，专门用来处理客户端连接事件，该方法中核心的目的是把针对SocketChannel的handler链表，添加到当前NioSocketChannel中的pipeline中。
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg;
child.pipeline().addLast(childHandler); //把服务端配置的childHandler，添加到当前NioSocketChannel中的pipeline中
setChannelOptions(child, childOptions, logger); //设置NioSocketChannel的属性
setAttributes(child, childAttrs);
try {
//把当前的NioSocketChannel注册到Selector上，并且监听一个异步事件。
childGroup.register(child).addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
if (!future.isSuccess()) {
forceClose(child, future.cause());
}
}
});
} catch (Throwable t) {
forceClose(child, t);
}
}pipeline的构建过程#
9.6.2节中，child其实就是一个NioSocketChannel，它是在NioServerSocketChannel中，当接收到一个新的链接时，创建对象。
@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = SocketUtils.accept(javaChannel());
try {
if (ch != null) {
buf.add(new NioSocketChannel(this, ch)); //这里
return 1;
}
} catch (Throwable t) {
logger.warn("Failed to create a new channel from an accepted socket.", t);
try {
ch.close();
} catch (Throwable t2) {
logger.warn("Failed to close a socket.", t2);
}
}
return 0;
}而NioSocketChannel在构造时，调用了父类AbstractChannel中的构造方法，初始化了一个pipeline.
protected AbstractChannel(Channel parent) {
this.parent = parent;
id = newId();
unsafe = newUnsafe();
pipeline = newChannelPipeline();
}DefaultChannelPipeline#
pipeline的默认实例是DefaultChannelPipeline，构造方法如下。
protected DefaultChannelPipeline(Channel channel) {
this.channel = ObjectUtil.checkNotNull(channel, "channel");
succeededFuture = new SucceededChannelFuture(channel, null);
voidPromise = new VoidChannelPromise(channel, true);
tail = new TailContext(this);
head = new HeadContext(this);
head.next = tail;
tail.prev = head;
}初始化了一个头节点和尾节点，组成一个双向链表，如图9-2所示
图9-2
NioSocketChannel中handler链的构成#
再回到ServerBootstrapAccepter的channelRead方法中，收到客户端连接时，触发了NioSocketChannel中的pipeline的添加
以下代码是DefaultChannelPipeline的addLast方法。
@Override
public final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) {
ObjectUtil.checkNotNull(handlers, "handlers");
for (ChannelHandler h: handlers) { //遍历handlers列表，此时这里的handler是ChannelInitializer回调方法
if (h == null) {
break;
}
addLast(executor, null, h);
}
return this;
}addLast#
把服务端配置的ChannelHandler，添加到pipeline中，注意，此时的pipeline中保存的是ChannelInitializer回调方法。
@Override
public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
final AbstractChannelHandlerContext newCtx;
synchronized (this) {
checkMultiplicity(handler); //检查是否有重复的handler
//创建新的DefaultChannelHandlerContext节点
newCtx = newContext(group, filterName(name, handler), handler);
addLast0(newCtx); //添加新的DefaultChannelHandlerContext到ChannelPipeline
if (!registered) {
newCtx.setAddPending();
callHandlerCallbackLater(newCtx, true);
return this;
}
EventExecutor executor = newCtx.executor();
if (!executor.inEventLoop()) {
callHandlerAddedInEventLoop(newCtx, executor);
return this;
}
}
callHandlerAdded0(newCtx);
return this;
}这个回调方法什么时候触发调用呢？其实就是在ServerBootstrapAcceptor这个类的channelRead方法中，注册当前NioSocketChannel时
childGroup.register(child).addListener(new ChannelFutureListener() {}最终按照之前我们上一节课源码分析的思路，定位到AbstractChannel中的register0方法中。
private void register0(ChannelPromise promise) {
try {
// check if the channel is still open as it could be closed in the mean time when the register
// call was outside of the eventLoop
if (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
}
boolean firstRegistration = neverRegistered;
doRegister();
neverRegistered = false;
registered = true;
//
pipeline.invokeHandlerAddedIfNeeded();
}
}callHandlerAddedForAllHandlers#
pipeline.invokeHandlerAddedIfNeeded()方法，向下执行，会进入到DefaultChannelPipeline这个类中的callHandlerAddedForAllHandlers方法中
private void callHandlerAddedForAllHandlers() {
final PendingHandlerCallback pendingHandlerCallbackHead;
synchronized (this) {
assert !registered;
// This Channel itself was registered.
registered = true;
pendingHandlerCallbackHead = this.pendingHandlerCallbackHead;
// Null out so it can be GC'ed.
this.pendingHandlerCallbackHead = null;
}
//从等待被调用的handler 回调列表中，取出任务来执行。
PendingHandlerCallback task = pendingHandlerCallbackHead;
while (task != null) {
task.execute();
task = task.next;
}
}我们发现，pendingHandlerCallbackHead这个单向链表，是在callHandlerCallbackLater方法中被添加的，
而callHandlerCallbackLater又是在addLast方法中添加的，所以构成了一个异步完整的闭环。
ChannelInitializer.handlerAdded#
task.execute()方法执行路径是
callHandlerAdded0 -> ctx.callHandlerAdded ->
-------> AbstractChannelHandlerContext.callHandlerAddded()
---------------> ChannelInitializer.handlerAdded
调用initChannel方法来初始化NioSocketChannel中的Channel.
@Override
public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
if (ctx.channel().isRegistered()) {
// This should always be true with our current DefaultChannelPipeline implementation.
// The good thing about calling initChannel(...) in handlerAdded(...) is that there will be no ordering
// surprises if a ChannelInitializer will add another ChannelInitializer. This is as all handlers
// will be added in the expected order.
if (initChannel(ctx)) {
// We are done with init the Channel, removing the initializer now.
removeState(ctx);
}
}
}接着，调用initChannel抽象方法，该方法由具体的实现类来完成。
private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
if (initMap.add(ctx)) { // Guard against re-entrance.
try {
initChannel((C) ctx.channel());
} catch (Throwable cause) {
// Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
// We do so to prevent multiple calls to initChannel(...).
exceptionCaught(ctx, cause);
} finally {
ChannelPipeline pipeline = ctx.pipeline();
if (pipeline.context(this) != null) {
pipeline.remove(this);
}
}
return true;
}
return false;
}ChannelInitializer的实现，是我们自定义Server中的匿名内部类，ChannelInitializer。因此通过这个回调来完成当前NioSocketChannel的pipeline的构建过程。
public static void main(String[] args){
EventLoopGroup boss = new NioEventLoopGroup();
//2 用于对接受客户端连接读写操作的线程工作组
EventLoopGroup work = new NioEventLoopGroup();
ServerBootstrap b = new ServerBootstrap();
b.group(boss, work) //绑定两个工作线程组
.channel(NioServerSocketChannel.class) //设置NIO的模式
// 初始化绑定服务通道
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel sc) throws Exception {
sc.pipeline()
.addLast(
new LengthFieldBasedFrameDecoder(1024,
9,4,0,0))
.addLast(new MessageRecordEncoder())
.addLast(new MessageRecordDecode())
.addLast(new ServerHandler());
}
});
}